1. Field of the Invention
The present invention relates generally to cellular communication systems. More particularly, the present invention relates to a recovery method to facilitate the completion of a soft handoff when a reverse link is faulty.
2. Description of the Related Art
FIG. 1 shows a schematic diagram of a conventional CDMA mobile communication system. As shown, the conventional system comprises: the base transceiver stations (BTSs) (30, 31) which provide the mobile communication services with the mobile stations 40, 41; the BSCs (Base Station Controller) 20, 21 which control BTSs 30, 31; the MSC (Mobile Switching Center) 10 which connects a plurality of the BSCs 20, 21 with the PSTN (Public Switched Telephone Network) 11.
The forward link is directed from the BTS to the mobile station, and the reverse link is directed from the mobile station to the BTS in the above CDMA system. All forward channels in each BTS share an identical PN (Pseudo random Noise) sequence offset. Therefore, the PN offset is an identification signal which distinguishes one BTS from another.
When a mobile station exits a present service region and enters a new region, the call is maintained by the handoff function in the cellular system. The CDMA mobile communication system provides various handoffs to guarantee the call continuity. The method for connecting a plurality of call paths without the need to switch frequencies by establishing a communication path with a new base station prior to terminating a communication path with an existing base station is called a soft handoff. U.S. Pat. No. 5,722,074 "Soft handoff in a cellular telecommunication system" discloses a soft handoff method in which the mobile station can communicate the wireless signals with at least 2 BTSs.
The system which supports the soft handoff manages a plurality of cells (or sectors), as an active set for connecting the mobile station to the channel. The mobile station changes the active set only by command of the BSC. That is, a new cell (or a sector) with a gradually increasing PN offset signal level is added to the active set, and the existing cell (or sector) with a gradually decreasing PN offset signal is removed from the active set.
FIG. 2 shows a conventional soft handoff process. When receiving a new PN offset signal, the mobile station 40 reports the change to the active set to the BSC 20 through the BTS (step 110). The BSC 20 determines if handoff is performed or not and the kinds of handoff (step 120) and commands the channel element of the BTS 30 to transmit a new wireless resource (step 130).
When the BSC 20 instructs the mobile station 40 to perform the handoff (step 140), the mobile station 40 changes the active set by performing the handoff, and reports the handoff completion to the BSC 20 through the BTS 30.
As previously stated, the handoff process initiated from the mobile station 40 is performed by command of the BSC 20.
In performing the handoff in the manner described above, when the quality of the 20 forward wireless link or the reverse wireless link is degraded because of the influences of the geographical features, artifacts, building, and climate, messages relating to the handoff may be lost. Further, the quality of the reverse link is equally likely to be degraded. When the HCM (Handoff Completion Message) of the mobile station 40 is lost (step 150) because of the poor quality of the reverse link, the BTS doesn't know whether handoff is completed or not, so it may occur that the handoff is not correctly performed afterwards. As a result, a discontinuity phenomenon of the call occurs because the mobile station is not controlled for proper handoff according to its direction of motion.